1. Field of the Invention
This invention concerns thermoplastic molding and extrusion compositions of an aromatic polycarbonate resin and copolymers of ethylene and acrylic or methacrylic acid. The compositions exhibit good impact strength after molding while showing less tendency to delaminate in comparison with their prior art counterpart compositions of polycarbonate and polyethylene.
2. Description of the Prior Art
The aromatic polycarbonate resins constitute a well know family of polyesters of carbonic acid which are prepared from dihydric or polyhydric phenols through a condensation reaction with a carbonate precursor. The polycarbonate resins may be processed with a wide variety of conventional methods, including injection or compression molding, sheet or profile extrusion, thermoforming, blow molding and blown film extrusion. The finished products are characterized by an outstanding profile of properties, such as high ductility and stiffness, good dimensional stability, high creep resistance, good electrical properties, low water absorption and good stain resistance. These polymers are also noted for their glass-like clarity or transparency, which together with other properties makes them very useful in medical-related applications such as sterile packaging films, intravenous feeding kits and blood processing equipment.
Descriptions of the aromatic polycarbonate resins and methods of their preparation are set forth in the patent literature, including U.S. Pat. No. 3,153,008 (Fox), U.S. Pat. No. 2,946,766 (Schnell, et al.), and U.S. Pat. No. 3,028,365 (Schnell, et al.), incorporated herein by reference, and elsewhere.
As in the case of other thermoplastic polymers, attempts have been made to admix or blend aromatic polycarbonate resins with other polymers to upgrade one or more chemical or physical properties, for purposes of fitting specific end use requirements and to achieve a spectrum of properties not found with the individual polymers. Some of the attempts and results are described in the patent literature.
For instance, U.S. Pat. No. 3,431,224 (Goldblum) states that aromatic polycarbonate resins are made more resistant to environmental stress crazing and cracking by admixture with 0.25 to 50 percent of a modifier polymer selected from among polyethylene, polypropylene, polyisobutylene, copolymers of ethylene and alkyl acrylate, copolymers of ethylene and propylene, cellulose esters, polyamides, polyvinyl acetal, alkyl cellulose ethers and polyurethane elastomers. The patent notes in column 2, on lines 41-47, that the modifier polymers may themselves be subject to crazing or cracking while under stressful contact with organic solvents and would therefore not be expected to improve these properties when combined with the polycarbonate resin, yet they do.
U.S. Pat. No. 4,397,982 (Boutni and Liu) discloses thermoplastic compositions comprising a polycarbonate resin, a linear low density polyolefin (for example, polyethylene) and a multiphase interpolymer of a C.sub.1-5 acrylate and a C.sub.1-5 methacrylate (for example, Rohm and Haas' Acryloid KM-330, consisting of n-butyl acrylate and methyl methacrylate). These compositions are described as exhibiting superior impact properties at temperatures as low as -30.degree. C., and in addition good weld line strength, lower melt viscosity and improved heat stability compared with the corresponding unmodified polycarbonate compositions.
While the efficacy of polyolefin resins as modifiers for polycarbonates has unquestionably been recognized in the prior art, practice has often shown that products molded from compositions of a polycarbonate and a polyolefin tend to delaminate, especially when the composition contains higher amounts of the polyolefin. This is evidenced, when the molded article is cross-sectioned, by the appearance of fissures or cracks along seams in the interior of the article, where the molten polymers have flowed together during the molding cycle but failed to fully and compatibly mix into a bomogeneous whole. The effect is often more pronounced when relatively high shear conditions are used for molding. This lack of compatibility between the two polymers often results in poorer impact resistance, as well as deficiencies in other properties.
The need exists, therefore, for a material which can be substituted for polyethylene and other polyolefins in polycarbonate compositions without loss of the beneficial properties that characterize such compositions, while also reducing or completely avoiding the unfortunate tendency toward delamination.